For MCP Streamable HTTP, POST each JSON-RPC message separately to the server endpoint. The client Accept header includes both application/json and text/event-stream. An accepted notification receives HTTP 202 with an empty body. A server that does not offer a server-to-client SSE stream answers GET with HTTP 405. Subsequent requests carry the negotiated MCP-Protocol-Version. Reject unsupported protocol-version headers with HTTP 400. Scope: MCP specification 2025-11-25. Reviewed September 19, 2026.